1. Field of the Invention
The present invention relates to a semiconductor device including a circuit constituted by thin film transistors (hereinafter referred to as “TFTs”). Particularly the present invention relates to the structure of, for example, an electro-optical device typified by a liquid crystal display device or an EL (electroluminescence) display device, a semiconductor circuit, and an electronic instrument (electronic equipment) using the electro-optical device or the semiconductor circuit of the invention.
Incidentally, the semiconductor device in the present specification indicates any devices functioning by using semiconductor characteristics, and all of the electro-optical device, the semiconductor circuit, and the electronic instrument are included in the semiconductor device.
2. Description of the Related Art
Since a thin film transistor (hereinafter referred to as a “TFT”) can be formed on a transparent substrate, development to apply it to an active matrix type liquid crystal display (hereinafter referred to as an “AM-LCD”) has been actively made. Since a TFT using a crystalline semiconductor film (typically, polysilicon film) can obtain a high mobility, it is possible to realize a high fineness image display by integrating functional circuits on the same substrate.
Basically, the AM-LCD includes a pixel region (also called a pixel matrix circuit) for displaying an image, a gate driving circuit (also called a gate driver circuit) for driving a TFT of each of pixels arranged in the pixel region, and a source driving circuit (also called a source driver circuit) or a data driving circuit (also called a data driver circuit) for transmitting an image signal to each pixel TFT, which are formed on the same substrate. Incidentally, a region where the gate driver circuit and the source driver circuit are formed is called a driver circuit region.
In recent years, there is proposed a system-on-panel in which in addition to the pixel region and the driver circuit region, a signal processing circuit such as a signal dividing circuit or a γ-correction circuit is also provided on the same substrate.
However, since performance required by a circuit is different between the pixel region and the driver circuit region, it is difficult to satisfy all circuit specifications through TFTs having the same structure. That is, under the present circumstances, such a TFT structure has not been attained that both of a TFT constituting the driver circuit region including a shift register circuit or the like in which importance is attached to a high speed operation and a TFT constituting the pixel region (hereinafter referred to as a “pixel TFT”) in which importance is attached to a high withstand voltage characteristic are satisfied at the same time.